530 



BOTANY OF THE LIVING PLANT 



Ferns, and generally in young sporelings there is a simple stele of a 

 type called a " prcitostele," havnig a solid xylem-core, and phloem 

 surroundmg it This is believed to have been the primitive structure 

 for them all. It is well shown in Bolryopteris (Fig. 268). Occasionally 

 this state may be retained through life [Hymeiw phylluin, Lygodium). 

 But in the vast majority of Ferns the stele expands as the plant grows 

 stronger, and the leaves larger ; and in various ways it becomes segre- 

 gated into a number of vascular strands [meristeles) , arranged in a 

 cylindrical network (Fig. 267 E, F). Each mesh in Nephrodiim 





Fig. 269. 

 Transverse section of rhizome of Bracken, omitting tlie external band of scleren- 

 chyma, and showing the outer and inner series of meristeles, and the irregular 

 hands of dark sclerenchyma between them. ( x lo.) 



corresponds to the insertion of a leaf -base, and is called a foliar mesh, 

 or gap. The vascular strands that run out into the leaf, called col- 

 lectively the leaf-trace, arise from the margin of it, and the cortex and 

 pith are in direct communication through the foliar gaps. Nephrodiiim 

 gives a comparatively simple example of this subdivision of the stele, 

 which is characteristic of most Ferns, and is to be regarded as 

 derived from the primitively undivided protostele. The leaf-trace 

 in different kinds of Ferns may be a single strand, or many. In either 

 case it sends out branches into the pinnae and pinnules, and they end 

 either m an open venation, as in Neplirodiiim or Plciidium (Fig. 266) ; 

 or in a network of veins as in the Adder's tongue. Thus the 



